REGISTER

GREGORY W.R. ROYLE

Manager Trusted Systems Computer Sciences of Australia Pty Ltd

Safety Auditing is an engineering activity which has arisen from requirements for the development of so-called "safety-critical systems". A safety-critical system is one in which a failure or design implementation error could cause risk to human life; it requires the highest level of safety integrity. Clearly, a railway signalling system fits this definition. The term "safety auditing" is introduced in the UK Defence Standards 00-55 and 00-56 (References 1 and 2). In these standards, an Independent Safety Auditor (ISA) has a role of assessing the safety of the overall system in a way which is free from any conflicts of interest. This role is referred to as an "Assessor" in other relevant standards such as IEC 65a (Reference 3) and FUA Spec. 23 (Reference 4).

Safety-critical systems must be developed in a way which ensures that the risk to human life during operation of the system is acceptably low. This is achieved by establishing a Safety Program to operate throughout the system's development.

A Safety Program comprises three main elements:

• safety program management
• safety analysis
• safety verification and validation (V&V).

Safety analysis involves hazard analysis, hazard tracking, risk assessment and hazard resolution. Safety V&V involves the use of personnel who are not involved in other aspects of the project to perform checking and testing of the safety elements of the systems development against the system specification and progressive specifications and design documents. Safety Program Management involves the organisation, planning, monitoring and review of all safety activities to ensure that they are performed satisfactorily. The role of the ISA is part of the Safety Program Management.

Although the Safety Program is identified as a separate element, it is part of the overall system development process. As such, it interfaces into, and works in conjunction with, other activities in:

• project management
• system engineering (including software and hardware engineering)
• system verification and validation.

As such, an important element of safety Program Management is ensuring effective integration of the safety engineering activities with the other system engineering activities, yet maintaining a focus on the safety issues to ensure that the safety objectives are not compromised.